The Climate System

EESC 2100 Fall 2007

The Earth's Radiation Budget, Part II.

I. Purpose

Last week you explored the geographical variations of Earth's albedo, reflected
solar radiation, and Earth's radiation received by satellites from cloud free
areas of the Earth's surface. The patterns you observed were controlled by
the curvature of the earth, variations in seasonal radiation received from
the sun, and varying properties of the Earth's surface. This week we will explore
the effect of clouds on these patterns. Accordingly, the data sets you will
first look at this week will be that under the category:
total
(this link will open a new window with these data).

We will want to compare some of the cloud
free data sets from last week's lab with the total datasets used this
week. These comparisons, which we recommend you do when looking at the albedo
data sets, will allow you to differentiate between reflectivity that is
caused by clouds and that which is caused by Earth surface properties such
as ice and snow. As you look at these data, note what areas of the Earth
clouds persistently cover and what areas are generally cloud free. The reasons
for these patterns will become clear as the course continues.

II. Examining the ERBE Data.

A. Total albedo

Go to the window you opened earlier that contains the total fields.
Look at the January map of total albedo (adjust your
graphical interface so continental outlines are shown, set your colorscale
range from 0 to 100 and then choose the option of "colors | contours").
Access clear-sky albedo from last week's lab in your other opened window
(make the same display choices as above).

Task I: Comparing between clear-sky and total.

Compare the clear-sky albedo to total albedo in January. What are
the effects of clouds on albedo? (Results) How
do you explain the changes in albedo due to clouds? (Discussion)

Compare total albedo in January with total albedo in July (seasonal
effects). Where are areas with persistent high albedo? low albedo? (Results) How
do these areas compare to areas of persistent high or low clear-sky
albedo? (Discussion)

B. Total short wavelength reflected radiation

Go back to the windows that contain the total and clear-sky data
and navigate to the shortwave fields. We will now calculate the globally
averaged amount of reflected solar radiation in the clear-sky case and
in the total to evaluate the effect of couds on the radiation budget.
To do that, click on the "expert mode" link in the upper
right corner of the window. We will first average all the data over time
to look at the annual average. In the expert mode window
type the following line of text below the text which is already present
in the window:

[T] average

Now click the "OK" button to the right of the expert window.
This tells the software to average the data over all time slices. Each
point in space is averaged separately. View this field to look at the
annual average total reflected shortwave radiation. Do the same with
the clear-sky field. As with the albedo fields, a comparison between
the total and clear-sky data highlights the role of clouds in the short
wavelength budget.

Now return to expert mode and continue typing underneath the first line
you entered:

Y cosd mul

This multiplies every grid point in space by the cosine of the latitude
angle (Y is the latitude angle in degrees, and cosd is a calculation
of cosine when the angle is given in degrees). We need to do that so
that our grid points will be properly weighted with respect to the geographical
areas they represent as there are more grid points per unit area in the
high latitudes than in the tropics. Then type:

[X Y] average

Cick the "OK" button again. The viewer will return a single
number just below the expert window (in bold letters). That number is
the amount of reflected shortwave energy averaged over the entire globe
in W/m2. Do the same operation with clear-sky shortwave radiation.

Task 2: Record the global annual averages for both the
total and clear-sky shortwave radiation. (Results) How
do you explain the difference between these numbers in relation to cloud
cover? If cloud cover increases, how will this difference change? (Discussion)

C. Total long wavelength Earth radiation

Open a new browser window with the total longwave
radiation dataset (hold the apple/command key down when you click
here). In the expert mode calculate the annual mean outgoing longwave
(that is, type "[T] average" and click OK). In the other window, go
back to the view of the global mean reflected radiation. Display the
figures side-by-side.

Task 3:

What is the overall relationship between the emitted
longwave and refected shortwave? (Discussion)

Use
the Stefan-Boltzmann equation (I = σT4) to calculate
the emission temperature of the earth at one of the areas of maximum
longwave radiation. (Results)

In areas of the tropics where longwave emission is low, the emissions
actually come from the top of deep convective clouds. Calculate the
temperature in these areas. (Results)

D. Total net radiation

The net radiation is the difference between the radiation coming into
the Earth from the sun and the energy radiated by the Earth to space.
For the planet as a whole what comes in must equal what goes back out;
however, more radiation comes in at the tropics than goes out in these
latitudes and more is radiated from the higher latitudes (north and south
of the tropics) than comes in. This difference provides the energy to
drive the circulation of the atmosphere and ocean.

Task 4: Calculate the global annual average net
radiation (use
the expert mode again). (Results) What percentage
of incoming solar radiation at the top of the atmosphere (So/4 =
342 Wm-2) is the global annual average
net radiation? (Results) (You should find one value
for global annual average net radiation using the same technique as in Section
B.)

E. Cloud Forcing

In order to try to understand how clouds affect the Earth's radiation
budget, ERBE scientists calculated the difference between the clear-sky
fields and the total fields. The result is often referred to as cloud-forcing.
The fields in this dataset show how much clouds affect the amount of radiation
available to Earth by comparing the data from the same locations during
cloudy and non cloudy days. This calculation cannot be done in some places
due to insufficient data.

Go to the annual
average net cloud forcing dataset. Here you can see that clouds may
locally warm or cool a given region. The degree of cloud cover at given
location is necessary but not sufficient to determine the cloud forcing.
Compare the region over the western tropical Pacific (near Indonesia and
northern Australia) to the North Pacific (between Japan and Alaska), two
areas with extensive cloud cover.

Task 5: What is the net cloud forcing in these two areas? (Results) How
does this relate to the balance between shortwave reflection and longwave
emission? (Discussion)

F. The Greenhouse Effect

The Stefan-Boltzmann Law relates the amount of longwave radiation emitted
by a black body to its temperature. Thus we can calculate the effective
temperature of Earth as determined by its total longwave radiation emitted
into space, and compare it with the surface temperature. To do that let
us go back to the total longwave radiation dataset (you
might want to close all other windows first).

In the expert mode we can calculate the temperature corresponding to
the outgoing longwave radiation by first dividing by the Stefan Boltzmann
constant and then taking the square root of the result twice. We do that
as follows:

5.67E-08 div
sqrt sqrt
273.15 sub
X Y 1 SM121

This code converts the results to °C and add some smoothing in space.
View the results in colors and contours.

Do the values agree with your notion of temperature
on Earth? Why are they so cold?

Compare the January field from the Jones
data to that calculated from ERBE. At the surface, temperatures generally
drop uniformly from equator to pole. This is not true about the effective
temperature as calculated from longwave radiation. Point at the outstanding
differences and try to explain them in light of all the material covered
in class and in this lab.

We can use these temperatures to calculate the amount of longwave radiation
emitted from the Earth's surface, and compare that to the ERBE measurements
of what is emitted into space. Go back to the longwave radiation dataset
and in the expert mode replace the Stefan Boltzmann calculation with
a calculation of the annual averaged longwave radiation using "[T] average"
as described before.
Then go to the Jones surface temperature dataset and use the Stefan Boltzmann
Law to calculate the longwave radiation emitted from the surface assuming
that surface emissivity is 1 (not accurate, but sufficient for our purpose).
To do that, type the following into the expert window:

273.15 add
dup mul dup mul
5.67E-08 mul
[T] average

The command "dup" duplicates the data which is then multiplied by
the original using "mul". This is done twice to create the 4th power of temperature
in K.

Task 7: Compare the annual averaged longwave radiation
coming from the surface with that going out to space. Which is larger? What
does the difference represent? Where is the effect (difference) largest.
Where is it smallest?

III. Hands-on Experiments

A. Black and white "blackbodies"

In the previous section, you used the Stefan-Boltzmann law to calculate
the effective temperature of a planet heated by the sun. The same reasoning
applies to ordinary paper heated by a lamp. In this experiment you will use
a desk lamp, a two channel thermometer and two pieces of paper, one black
and one white. Both pieces of paper can be treated as black bodies with different
albedos. Calculated albedos are written on the pieces of paper. You will
do the experiment on both pieces of paper at the same time under the same
conditions. Tape one of the thermometer sensors to each piece of paper. Place
the papers close to each other under the desk lamp and turn the lamp on.
You will find that the temperatures of both papers immediately rise. Why?
After a few minutes, the temperatures stop growing and stay constant, which
means the papers have reached an equilibrium state. Now write down those
steady state temperatures. Why is there a big difference? Use the Stefan-Boltzmann
law to calculate the energy emitted by the lamp in two ways: one using the
thermal equilibrium of the white paper, the other for the black paper. Are
they the same?

B. Radiative energy spectrum from different light sources:

It is often very useful to describe a light-emitting body in terms of its
emission spectrum, that is, the partitioning of energy among the different
frequencies (or wavelengths) composing the light. For example, the sun emits
most of its energy in the visible part of the spectrum, centered on wavelengths
around 500 nm (green). This means that its emission spectrum shows a bump
at these wavelengths, where its brightness is at a maximum. A spectroscope
is a device that allows you to see the spectrum of the incoming lights. Point
the spectroscope at two different light sources, a fluorescent light (ceiling
light) and a halogen light (desk lamp). You will notice that the different
colors are not present at the same brightness. Sketch a qualitative spectrum
plot (intensity versus wavelength) for each light sources. Make sure to label
both axes and use the spectral chart to convert the colors to wavelengths
in microns. Compare the two spectra. Are both continuous? Where are peak
intensities?

Write a lab report (as per the Lab Report Format) summarizing the major findings
of your investigation. In addition, explain the following questions in your
discussion section:

Discuss the effect of clouds on the earth's radiation balance. Include
in your answer their net (overall) effect, their regional and seasonal
effects (give examples) and how different types of clouds in different
regions affect the balance.

Discuss the greenhouse effect as it is expressed in the difference between
the longwave radiation emitted from the Earth's surface and that emitted
from the top of the atmosphere. In answering this question relate to the
annual-average picture. Address regional differences and explain what factors
govern variations in the greenhouse effect as best as you can from the
material you have studied in class and in the lab.

Report the results of your hands-on experiments.

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